HISTORICAL ICE-MARGIN LOSS OF SELECTED GLACIERS IN THE BEARTOOTH MOUNTAINS OF MONTANA: EPISODIC OR CONTINUOUS?

Historical aerial and surface photographs indicate dramatic ice-margin loss in small cirque glaciers of the Beartooth Mountains in Montana during the last one hundred years. The Grasshopper and Castle Rock Glaciers lost over half their surface areas between 1898 and 1952, and 57% and 52% of their remaining areas between 1953 and 2001. Ice-margin loss for these glaciers between 1898 and 1953 was approximated by comparison of historical ground photos of glacier margins with annual temperature and precipitation data provided by recording stations at lower elevations (6200-7540 ft.) located 10-30 miles from the glaciers. Ice-margin loss for this interval was strongly episodic, with high winter precipitation and low summer temperatures maintaining the glacier margins from 1898-1932.

Timing of ice-margin loss between 1953 and 2001 was determined by direct comparison of LANDSAT 1-5, NAHP and NAP aerial photos of the glacier margins with annual summer (June-September) temperature and winter (October-May) precipitation data provided by three SNOTEL stations at higher elevations (8700-9280 ft.) located within 5-10 miles of the glaciers. Ice-margin loss for this interval was episodic, with high winter precipitation offsetting the impact of high summer temperatures for glacier margins from 1953-1982.

Results of the study suggest that the major episodes of ice-margin loss were 1933-1937 and 1983-1988, with minor events during 1939-1941, 1953-1956, 1960-61, 1990-1991, 1994, 2000-2001, and 2003. The most severe episode was 1933-1937, where prolonged record summer temperatures followed five consecutive winters of record low precipitation.